Composite titanium-alloy article resistant to hot salt corrosion



United States Patent M 3,434,813 COMPOSITE TITANIUM-ALLOY ARTICLE RESIST- ANT TO HOT SALT CORROSION Howard B. Bomberger, Jr., East Liverpool, Ohio, assignor to Crucible Steel Company of America, Pittsburgh, Pa., a corporation of New Jersey No Drawing. Filed Dec. 7, 1964, Ser. No. 416,649

Int. Cl. C01g 23/00 US. Cl. 29194 2 Claims ABSTRACT OF THE DISCLOSURE This invention relates to titanium-alloy articles resistant to corrosion when exposed to salt at a moderately elevated temperature, such as about 500 to 900 F. One shortcoming of known titanium-base alloys is that when heated to such temperatures in the presence of salt, they tend to embrittle, lose strength, and ultimately fail. Small amounts of salts, as from ocean spray, chlorinated solvents, or even fingerprints, can cause this phenomenon. Particularly in supersonic aeronautical applications, both the exterior and the interior parts of the craft are likely to encounter the combination of conditions, i.e., salt, stress, and elevated temperature, that causes trouble. Accordingly, it is an object of my invention to provide titanium-base alloy articles, such as sheets, strip, bars and forgings, that overcome this difliculty. This and other objects will be apparent from the following description.

In brief summary, I have discovered that by providing a titanium-base alloy article with a coating, cladding, or surface layer containing a large proportion, or consisting entirely (except for impurities), of zinc, I obtain an article that resists salt corrosion and retains substantially the same strength and ductility during long-term exposure (several hundred hours) to moderately elevated temperatures (about 500 to 900 F.) that the uncoated alloy would exhibit in the absence of salt. Surpisingly, even a discontinuous zinc coating has been shown to be effective. In a preferred mode of practicing the invention, a highstrength titanium-base alloy, i.e., one exhibiting a roomtemperature yield strength of 110,000 p.s.i. or greater, is clad, electroplated, hot-dip coated, or flame-spray coated with commercially pure zinc or a suitable alloy thereof. Certain metals such as mercury and cadmium embrittle titanium and its alloys, and such metals must be excluded from the coating-metal composition used in the practice of my invention.

The substrate material used in practicing my invention will, in general, be selected from the titanium-base alloys having a room-temperature yield strength at 0.1% offset of 110,000 p.s.i. or more. Commercially pure titanium, with a room-temperature yield strength of about 40,000 to 70,000 p.s.i., lacks the strength necessary to suit it 3,434,813 Patented Mar. 25, 1969 for use at moderately elevated temperatures. Hence, the invention is used with titanium-base alloys of any of the known microstructural types (alpha, beta, or mixed), such as Ti-5Al-2.5Sn, Ti8Al-1MolV, Ti-13Vl1Cr 3A1, and Ti-6Al-4V. More likely than not, the substrate will be used in a fiat-rolled product form, such as strip, sheet, or plate, but the invention is also useful with bars, rods, wire, forgings, and other product forms.

As will be apparent to those skilled in the art, the coating or cladding material used in my invention will depend upon a number of factors, but most importantly the intended end use of coated or clad article. For uses not involving exposure to temperatures over about 700 F., the use of a coating or cladding of commercially pure zinc is preferred. For higher service temperatures, such as up to about 1000 F., it is preferred to use either titanium alloy with an interdiffused surface layer or zinc or a coating or cladding of zinc-base alloy such as 5OAl- 50Zn. For most purposes, it is desirable but not essential that the coating remain solid at the temperature of use, but in certain applications satisfactory results are obtained at temperatures above the melting point of the coating.

Various methods can be used to apply a coating or cladding, or produce an interditfused zinc surface layer, in accordance with the invention. These methods include hot dipping, electroplating, flame-spray coating, and others. The procedures required for obtaining adherent coatings of zinc by hot dipping or electroplating are well known in the art, as is also a suitable flame-spray coating procedure. Cladding either by cold rolling zinc sheet in contact with titanium alloy or rolling a composite piece produced by casting zinc around or on one side of a titanium-base alloy sheet may be used. An interdiifused zinc surface layer may be produced by confining the part to be treated in an autoclave or bomb, along with a suitable quantity of zinc, heating to a temperature such as about 1200 C., i.e., above the boiling point of zinc, and holding for a suitable time, such as 2 hours, to produce a layer of desired thickness, such as about 0.05 inch. For certain purposes, surface layers of considerably smaller thickness, e.g. about 0.0005 inch, which can be produced in a considerably shorter time, will prove adequate.

The present invention may be illustrated by means of the following examples.

EXAMPLE I Standard creep-test specimens were prepared of metal having the following composition, the numerals representing weight percent: 1

Al 4.9 Sn 2.5 Fe 0.15 N .02 C .05 H .012 Ti Balance The specimens were flame-spray coated with zinc metal to a thicknes of 0.011 inch, and then provided with a salt coating about 0.050 inch thick. This was done by alternately swabbing the gage length of each specimen with a saturated aqueous solution of sodium chloride and then drying in a warm (about F.) oven, about three repetitions of these operations being required to afford a salt coating of the desired thickness. The specimens were creep-tested for 313 hours at 750 F. with an applied force of 45,000 p.s.i., the observed creep being 0.02%. The creep specimens were then tensile-tested, and they exhibited the properties presented in the following table, which also presents the results with similar creep specimens that were not provided with a zinc coating.

TABLE I Yield Protective Coating Ultimate strength Elonga- Reduccoating thickness, tensile at 0.2% tion, tion of in strength, oflsct, percent area,

p.s.i. p.s.i. percent From the foregoing table, it can be seen that use of the flame-sprayed zinc coating made it possible to avoid embrittlement and retain good ductility properties. Visual examination of fractured specimens revealed that the uncoated specimens were penetrated by the salt to a depth of 0.020 inch, whereas the salt did not penetrate the zinc-coated specimens.

Further data, presented below in Table II, demonstrate that the invention can be used for the protection of the commercial Ti-6Al-4V and Ti-Al-2.5Sn alloys, both with the coating molten and with the coating in the solid state. The composition of the aluminum-tin alloy was as stated above. The composition of the aluminum-vanadium alloy was as follows, the numerals representing weight percent:

A1 6.3 V 4.0 Fe 0.16 N 0.023 C 0.02 H 0.004 Ti Balance Creep-test specimens having a diameter of inch and a gage length of 1 inch were prepared from the above alloys, coated wtih zinc or not as indicated below, coated with salt in the manner mentioned above, creep-tested as indicated below, and then tensile-tested. All coatings had a nominal thickness of 0.010 inch.

TABLE II Creep test Alloy Coating Temp., Time, Stress, Creep,

F. hr. p.s.i. percent 52.5 Zn spray 750 313 45, 000 0.0 6-4 do 750 313 45,000 1.0 750 313 45, 000 0. 0 6-4 do 1 750 313 45, 000 0. 6 52.5 Zn Dip 750 300 40, 000 0. 0 6-4 do 750 300 40, 000 0. 5 900 313 15, 000 0.0 900 313 15, 000 3. 5 900 200 20, 000 0. 01 900 200 10,000 0. 1 1, 000 110 5, 000 0. 0 1, 000 110 5, 000 1. 5 750 313 45, 000 0. 0 900 200 20, 000 0. 05 6-4 do 750 313 45, 000 0. 7

1 Cloating was repeatedly scratched with knife blade to expose base m a 2 Coating was applied by dipping in zinc at 1100 F for minutes.

Tensile test Yield strength at 0.2%

offset,

p.s.i.

Alloy Coating Ultimate strength,

tion, percent Zn Spray 137, 700

Coating was repeatedly scratched with knife blade to expose base m1? C iiating was applied by N.D.-Not determined.

From Table II, it can be seen that alloys not protected with a zinc coating were severely embrittled, exhibiting an elongation of under 5% in one inch and a reduction of area under 12%; in contrast, in each case, where the alloy was provided with a protective zinc coating it retained an elongation of over 12% and a reduction of area of over 30%.

Although in the foregoing description of the invention reference has been made specifically to common salt (sodium chloride), it is known that other chlorides are similarly very harmful to the properties of titanium and its alloys at temperatures above about 500 F., and a similar detrimental effect is observed wth certain other halides and sulfides. It is considered that high-temperature corrosion of titanium and its alloys caused by these other salts may be avoided by means of the practices disclosed above.

While I have shown and described above several embodiments of my invention, I intend to cover as well any change or modification therein which may be made without departing from the spirit and scope of the invention.

I claim:

1. A metal article exhibiting good strength and ductility after prolonged exposure at moderately elevated temperature to environments containing substances exerting an embrittling influence on titanium and titanium-base alloys, said article comprisig a body consisting essentially of a titanium-base alloy selected from the group consisting of Ti-5Al-2.5Sn, Ti-8Al-1Mo-1V, Ti-l3V-11Cr-3Al, and Ti-6Al-4V, said titanium-base alloy body being provided with a surface layer of metal selected from the group consistnig of zinc and zinc alloys.

2. An article as defined in claim 6, characterized in that said separated portions of zinc are interdiifused into said titanium alloy to form said surface layer.

References Cited UNITED STATES PATENTS OTHER REFERENCES WADC Technical Report 52-313, part I. November 1952, Brazing Titanium to Titanium and to Mild and Stainless Steels, pp. 5-7.

HYLAND BIZOT, Primary Examiner.

US. Cl. X.R. 29-198 dipping in zinc at 1,100 F. for 15 minutes. 

